LONG-TERM CHERNOBYL FALLOUT: COMPARISON TO BOMBS ALTERED

By STUART DIAMOND

Published: November 4, 1986

IN the aftermath of a dramatic assertion in September that the amount of long-term cesium radiation emitted into the environment from the Chernobyl disaster equaled that of all previous nuclear bombs and tests, a number of scientists have concluded that the assertion was wrong.

Although they concur in the conclusion that the Chernobyl long-term radiation, primarily cesium-137, was significant, the scientists say it was not equal to all other cesium-137 emissions. They said the actual amount of cesium-137, the primary long-term component in the Soviet reactor's fallout, is probably in the range of one-tenth to one-sixth of that produced by the more than 400 nuclear devices whose radiation entered the atmosphere between 1945 and the present. Emphasis on Range

The calculation was made by Dr. Lynn R. Anspaugh, a biophysicist at the Lawrence Livermore National Laboratory of California, in a presentation to an American Nuclear Society symposium in Saratoga Springs, N.Y., on Sept. 17, and in two subsequent interviews with The New York Times. The Times published his comments on Sept. 23.

The next day, Dr. Anspaugh said that upon further reflection he would place more emphasis on the estimated range of the long-term radiation emitted at Chernobyl but that the article was not incorrect in reporting the higher figure put forth at Saratoga. In an interview yesterday, Dr. Anspaugh said, ''The comparison I made could have been more accurate,'' He admitted that he was off by 10 times in figures he presented at Saratoga Springs, but he insisted that in the context this was not a mistake since ''I was making a very rough calculation.''

Dr. J. W. Thiessen, deputy director of the Energy Department's Office of Health and Environmental Research, which is financing the Chernobyl radiation study in which Dr. Anspaugh is taking part, said: ''He admits that his figure was not correct. It was a back-of-the-envelope calculation.''

Dr. Charles DeLisi, director of the research office, said Dr. Anspaugh's ''best estimates now are an order of magnitude different from his best estimates then.''

It is unclear just how much difference the error makes in terms of predicting increased cancer rates or other adverse health effects. For one thing, nearly all the long-term radiation from Chernobyl was concentrated on populated land, whereas nearly all the bombs have been exploded over water or other sparsely populated areas. As a result, the effects of the Chernobyl and weapons radiation on humans are likely to be closer to each other than the raw fallout figures indicate, Dr. Thiessen and others said.

In addition, many scientists agree that Chernobyl deposited more cesium on the ground in some parts of Germany than all previous weapons tests had. And as much as half the cesium-137 from bombs has already decayed into harmless material. The isotope has a 30-year half life, meaning that every 30 years, half of it degrades into benign substances; it takes 200 years to decay by 99 percent. Most of the large atmospheric bombs were exploded 20 or more years ago.

Some scientists said, too, that whether the radioactive cesium from Chernobyl equaled the cesium in 400 nuclear bombs or only 40 may be of little importance. ''You draw the same conclusion, that this was a very significant event,'' said Dr. Jan Beyea, a nuclear physicist and consultant with the National Audubon Society.

The error has underscored the difficulty of getting accurate information to the public in a developing, technical event where key information is lacking, scientists said. The problem occurred during the 1979 Three Mile Island accident and at other times concerning the Chernobyl accident.

''This is a complex subject with enormous uncertainties; you are bound to get a lot of opinions stated as fact,'' said Dr. DeLisi, a biophysicist. ''You are bound to get confusion.'' Dr. Anspaugh, head of the Environmental Sciences Division at Lawrence Livermore, used Chernobyl fallout figures developed by a team assessing the accident for the Energy Department. He said the team used Soviet data and also worked backward from data obtained by worldwide radiation monitors after the accident. By far the most signficant long-term radioactive element from Chernobyl was cesium-137, he said. Strontium-90, an important element in weapons fallout, was present in only small amounts in Chernobyl fallout, the biophysicist said.

The study team came to the conclusion that the amount of cesium-137 released by the accident was one million to six million curies, he said. A curie is a measure of radiation.

A typical 20-kiloton atomic bomb releases 4,000 curies of cesium-137, he said at Saratoga Springs. He told the audience of scientists that all previous nuclear-fission bombs and tests totaled about a thousand 20-kiloton bombs, or 20 megatons, for a total cesium output of four million curies from bombs, compared with the Chernobyl range of one million to six million curies.

But the correct total for fission bombs and tests is about 217 megatons, 10 times the figure presented by Dr. Anspaugh, according to Dr. Thiessen and others. And the actual number of curies of cesium-137 is 34 million, not four million, from bombs and tests, said Dr. Thiessen, a radiation expert and a physician. That would mean that Chernobyl emitted about a tenth as much cesium-137 into the environment as all bombs and tests combined. 'Factor-of-10 Difference'

Dr. Anspaugh now agrees that there is a difference of a factor of 10. ''It is very clear that the correct figure is 200, not 20,'' he said yesterday, referring to total megatons of bombs and tests. If Chernobyl released six million curies of cesium, it would be about a sixth as much as previous bombs and tests; most Western scientists interviewed said the figure of one million curies was too low.

The biophysicist said his talk at Saratoga Springs was intended to show that even at cesium emission levels equal to all nuclear bombs and tests, Chernobyl will not produce a worldwide public health disaster in excess cancers, since weapons and tests have not done so. Those conclusions are disputed by some nuclear critics.

Atmospheric tests were banned by the United States and the Soviet Union by treaty on Aug. 4, 1963. Since then there have been 22 atmospheric nuclear tests by China and 26 by France, according to the Defense Department.

Dr. Beyea said he was concerned that ''misinformation was propagated'' by Livermore, a respected national laboratory, and that the lab did not publicly correct the mistake.

In an interview Friday, Norris P. Smith, chief spokesman for the laboratory, said he still had no reason to believe that Dr. Anspaugh was incorrect. ''It is not my job to insert myself into the question of truth or falsehood'' concerning a scientist's research, he said. ''If a scientist has something to say, we give him a forum to say it.''